1. Field of the Invention
This invention relates to a multilayer ceramic chip capacitor.
2. Prior Art
Multilayer ceramic chip capacitors have been widely utilized as miniature size, high capacitance, high reliability electronic parts, a number of such capacitors being contained in a single electronic equipment. In accordance with a recently increasing demand for smaller size, higher performance electronic equipment, multilayer ceramic chip capacitors also encounter a more rigorous demand toward smaller size, higher capacitance, lower cost, and higher reliability.
The multilayer ceramic chip capacitors are generally fabricated by layering an internal electrode-forming paste and a dielectric layer-forming paste by sheeting, printing and similar techniques followed by concurrent firing for integration.
Generally the internal electrodes are of conductors such as Pd and Pd alloys although expensive palladium is partially replaced by the use of relatively inexpensive base metals such as Ni and Ni alloys. Since internal electrodes of base metals are oxidized if fired in ambient air, the dielectric layers and internal electrode layers must be co-fired in a reducing atmosphere. Firing in a reducing atmosphere, however, causes the dielectric layers to be reduced, resulting in a lowering of resistivity. Non-reducible dielectric materials are thus proposed. Multilayer ceramic chip capacitors using non-reducible dielectric materials, however, have problems including a short life of insulation resistance (IR) and low reliability.
When the dielectric material is subject to a DC electric field, there arises another problem that its relative dielectric constant .epsilon..sub.s lowers with time. If thinner dielectric layers are used in order to provide chip capacitors of a smaller size and greater capacitance, application of DC voltage across the capacitor causes the dielectric layers to receive a more intense electric field, resulting in a more remarkable change of dielectric constant .epsilon..sub.s with time, that is, a more remarkable change of capacitance with time.
Capacitors are also required to have good DC bias performance. The term DC bias performance used herein is a rate of change of the capacitance of a chip capacitor from the capacitance with an AC electric field applied thereto to the capacitance with an overlapping DC electric field applied thereto. The capacitance generally decreases as the applied DC electric field is increased. Capacitors with poor DC bias performance have the problem that when a DC electric field is applied across the capacitors during normal operation, the capacitors lower their capacitance significantly to below the required capacitance.
The EIA standards prescribe the standard known as X7R characteristic that a rate of change of capacitance should be within .+-.15% (reference temperature 25.degree. C.) over the temperature range between -55.degree. C. and 125.degree. C.
One dielectric material known to meet the X7R characteristic is a composition of the BaTiO.sub.3 +SrTiO.sub.3 +MnO system disclosed in Japanese Patent Application Kokai (JP-A) No. 36170/1986. This material, however, experiences a great change of capacitance with time under a DC electric field, for example, a capacitance change of -10% to -30% when a DC electric field of 50 volts is applied at 40.degree. C. for 1,000 hours, failing to meet the X7R characteristic.
Other non-reducible dielectric porcelain compositions include the BaTiO.sub.3 +MnO+MgO system disclosed in JP-A 71866/1982, the (Ba.sub.1-x Sr.sub.x O).sub.a Ti.sub.1-y Zr.sub.y O.sub.2 +.alpha.((1-x)MnO+zCoO)+.beta.((1-t)A.sub.2 O.sub.5 +tL.sub.2 O.sub.3)+wSiO.sub.2 system disclosed in JP-A 250905/1986 wherein A is Nb, Ta or V and L is Y or a rare earth element, and barium titanate having added thereto Ba.sub..alpha. Ca.sub.1-.alpha. SiO.sub.3 in vitreous state disclosed in JP-A 83256/1990. These dielectric porcelain compositions could not meet all the requirements including good temperature dependence of capacitance, a minimized change of capacitance with time under a DC electric field, good DC bias performance, and a long accelerated life of insulation resistance. For example, the compositions of JP-A 250905/1986 and 83256/1990 have a short accelerated life of insulation resistance.